Why do Macs with T2 chips display a black screen so long?

The other day, I was in a hurry, but needed to restart my iMac Pro which had just completed cleaning up my Time Machine backups. This has a T2 chip, and FileVault is active on its internal SSD. It started up briskly, I logged in, then it showed a black screen for so long that I gave up waiting for it and got on with other tasks. The same happens whenever it updates its firmware: the display goes black, and it plays dead for at least a couple of minutes. The temptation to force it to shut down steadily increases, until at long last it shows signs of life again.

If you have a Mac, even one as blisteringly fast as an iMac Pro or a high-end mini or MacBook Pro, these black screens are always worryingly long. So why do they occur, and what can you do about them?

I have described in some detail the main stages of the boot process, which are summarised in the diagram below.


When a Mac with a T2 chip is starting up in its standard full security mode, once the user has entered their password in the login screen to give access to the startup disk, the T2 has to compare the signature of its boot loader and other components against a value saved in its NVRAM. If they don’t match, integrity information may be needed to be downloaded from Apple before deciding whether to reinstall macOS or lower the security level. During those checks, there’s a phase in which the Intel side of the Mac has to be shut down, preventing any graphics display – hence the black screen, as the display is driven from the main chipset, not the T2.

This situation is even more critical when the firmware of the T2 chip is to be updated. This involves downloading it direct from Apple, checking it, and its installation. During much of this period in the update, the Intel side has to be shut down, providing the user with just a black screen to gaze at.

The snag is that every Mac user has learned the meaning of a black screen: that Mac has been shut down, or has frozen and can’t recover except by rebooting. For the first thirty seconds or so, the black screen doesn’t cause any anxiety. But as time passes, it becomes more suspiciously the result of catastrophe. Hearing of many T2 Macs which have managed somehow to brick themselves with a firmware problem, it’s natural that the user should come to suspect that’s the cause of the continuing black screen.

Worse still is the fact that the only thing you can do is press the Power button to force your Mac to shut down, which is one action with the highest risk of making your problems far worse. At best it will get you nowhere, as you’ll then need to start your Mac up again, and face the same black screen purgatory. At worst, if that were to be done during a firmware update, it could damage the update, and then you’ll be nervously trying to perform a firmware restore, and worrying whether your Mac is now bricked.

So the only sensible course is to leave your Mac alone, staring alternately at its black screen and your watch. One minute passes, then another. Just how long should take, and how can you tell if something has gone wrong, and your Mac is now frozen?

The answer is, as long as the Intel side of your Mac is shut down, you can’t tell the normal from the broken. All you can do is cross your fingers and wait expectantly. Another minute passes. Surely it doesn’t take this long, does it?

Yes it can. Leave your Mac alone, have a drink, or a meal, and come back a bit later, by which time it should have completed its Secure Boot process, and maybe even opened the Finder.

We all know that a black screen means that a T2-equipped Mac has shut down its Intel chipset to update or check its firmware. However necessary, it’s a frankly terrifying failure of human interface design. For the user, it’s the most insecure boot imaginable.